This invention relates to a method for determining the amount of deformation induced in a material in response to a compressive force. The invention may be used to determine the extent and uniformity of asphalt compaction in a road building operation, or the stress patterns present in a gasket formed from elastomeric material.
Methods for determining the extent to which a material is deformed in response to a compressive force are known in the prior art. Such methods find application in fields as varied as road construction and in tests for determining the ability of compressed gasket to maintain a fluid-tight seal. In the case of road construction, these methods perform the important function of indicating whether or not the granular asphalt that is compressed by a steam roller over a roadbed has been effectively compacted into a road structure having a uniform asphalt density and thickness over its surface, as nonuniformities in thickness and density result in a road that is undesirably uneven and that might include void spaces. Such unevenness and void spaces can result in rutting, and can also collect water within the road structure that can turn into ice and create pot holes. In the case of gaskets, such method are used to estimate the uniformity and the intensity of the strain imposed on the gasket material by the bolts or other structures which serve to compress the gasket between two surfaces to create a sealing interface. Such knowledge of the strain patterns can in turn be used to estimate the effectiveness of the resulting seal, and even the expected wear pattern and life expectancy of the gasket.
Unfortunately, there are a number of shortcomings associated with the testing methods used in the prior art which significantly undermine their usefulness. For example, in the case of roadmaking, the compaction and thickness of the granular asphalt used to build the road may be estimated by extracting a number of random core samples over the newly constructed road. Needless to say, such a sampling technique destructively creates holes in the new road's surface which need to be repaired. Additionally, such testing may well fail to inform the road builders of the existence of one or more faulty areas in the road structure, since the information relied upon is confined to only a few randomly taken samples. Of course, the relative accuracy of such prior art testing could be improved by taking a larger number of samples. However, more extensive sampling would involve a concomitant increase in the damage to the road, and the time and expense associated with such testing. To avoid the problem associated with testing based upon core sampling, testing techniques based upon the irradiation of the road with a neutron source are also known. In such tests, the density of the asphalt is determined by measuring the amount and types of secondary radiation that results from the initial neutron radiation. However, such testing requires the use of expensive equipment and skilled operators, and utilizes potentially hazardous radiation. Also, since this testing can only be performed on a sampling basis, it is possible that large faults in the road may be missed.
In the case of gaskets, there are no known, direct nondestructive testing procedures which would allow one to accurately determine the strain patterns that result in such a gasket as a result of the compressive forces applied to it by the surrounding clamping structure. However, since it is known that the compressive load applied over the gasket should be as uniform as possible to avoid the development of premature leaks in the resulting gasket seal, attempts are routinely made to apply as uniform a clamping force as possible between the gasket and the surfaces that it seals. For example, where this clamping force is applied by a plurality of bolts, the person installing the gasket may attempt to equilibrate the compressive load on the gasket by applying an equal amount of torque on each bolt. Unfortunately, because there are many variables in the relationship between the amount of torque applied to a bolt and the resulting compression (such as thread imperfections, the presence of dirt or lubrication in the threads, etc.) the application of equal amounts of torque on the bolts may not result in a corresponding pattern of equal strain on the gasket.
Clearly, there is a need for a nondestructive method that accurately and reliably determines the amount of deformation induced in a material in response to a compressive load. Ideally, such a method would be able to determine the amount of compressive present in the material over the entire surface of the material, rather than a relatively few, isolated sample points so that the presence or absence of significant areas of nonuniformity in the deformation could be easily detected and corrected. It would further be desirable if such a method could be quickly and reliably applied to both large open surfaces, such as an asphalt road under construction, as well as to small surfaces of relatively difficult accessibility, such as an operational gasket compressed between two opposing surfaces. Finally, it would be desirable if such a test were not confined to only a few specialized materials, but was universally applicable to a broad spectrum of materials used not only in road building or gaskets, but virtually any engineering application that involved the application of a compressive force to a material.